THE DEVONIAN PERIOD. 461 



berry 1 describes the relics as fragmentary and worn, and suggests 

 that they may be the undigested residue of innumerable feasts of the 

 larger fishes upon their weaker kin. It is possible that there were 

 restricted tracts or passageways of the shallow sea through which 

 the inferior fish habitually passed for reasons connected with the depths, 

 currents, tides, food-distribution, or other sea conditions, and that in 

 so doing they were the victims of persistent attacks by their superiors, 

 and thus the bottom became strewn with the debris of the slaughter. 

 At the present time, sudden changes of temperature due to the shift- 

 ing of currents destroy great numbers of fish simultaneously and prob- 

 ably give rise to great fish deposits. Were it not for the fragmentary 

 condition of the relics of the ancient bone beds this would seem to 

 offer the most plausible explanation. 



The fragmentary nature of the remains makes a satisfactory iden- 

 tification and classification difficult and not a little revision of those 

 first made has been found necessary; still further changes will doubt- 

 less yet be required. What is said about kinds and classes must there- 

 fore be taken with some reserve. It is clear, however, that an extinct 

 order of fishes, arthrodirans (joint-necked), formed a signal feature 

 of the fauna; that sharks of several orders also constituted a large 

 element; that certain of the ancient ganoids (crossopterygians), were 

 present, and doubtless other forms. The arthrodirans, whose necks 

 were so joined to their bodies as to give their heads vertical motion, 

 a rare feature in fishes, were quite remarkable forms, and perhaps 

 held the leading place. Among them were the Macropetalichthys, 

 whose skull is illustrated in Fig. 208, a, and Dinichthys (terrible fish) 

 which rose to monster sizes a little later (Fig. 209, p. 463) and then 

 rapidly declined. The sharks embraced not only forms armed with 

 cutting and piercing teeth, but also forms provided with pavement 

 teeth for crushing shell-fish. The Teleostomi were of the ancient type 

 (ganoids, chiefly crossopterygians) which were usually characterized 

 by cartilaginous skeletons and bony scales, in contrast to modern 

 teleosts, which are characterized by bony skeletons and membranous 

 scales. In general the fishes of the time seemed to have been more 

 fully clothed with spines and defensive armor than modern fishes. 

 Compared with existing species they were doubtless heavy and clumsy 

 in movement, and sluggish and brutal in habit. 



From the degree of adaptation to sea conditions already attained, 



1 Paleozoic fishes of North America, Mono. XVI, U. S. Geol. Surv., 1889, p. 30. 



